This invention is an improvement in scanning light source photographic printers used to make reproductions on film or paper from transparencies. Photographic prints are usually produced with the objective of employing the full tonal scale of the reproducing emulsion, in order to render a visually pleasing image. To this end, a variety of black and white photographic materials are available for choice, such as papers exhibiting graded or variable contrast, as well as long-scale, low-contrast or short-scale, high contrast films. Additionally, the contrast of such materials can be further controlled by appropriate adjustments to time and temperature during the processing development step.
The operation of print exposure determination is usually one in which the density range existing in the original is matched, by some means, to the contrast limits of the reproducing medium so that the intensity-time product (I.times.T=E) of light passing through the original exposes the desired high and low density points on the reproducing emulsion. Pictorial and artistic photography, particularly as performed by professionals, usually allows for experimental adaptation and the choice of emulsion contrast and exposure time. However, in the fields of scientific, industrial, medical, military and photogrammetric photography, among others, it is impractical for the darkroom technician to attempt to assess the individual characteristics of each original photograph in a situation where thousands of images may have to be copied uniformly.
Scanning light source photographic reproduction devices, specifically those of the type called d-c coupled dodging printers, provide automatic exposure control by compensating for local density differences in the original transparency. As the light source scans a particular point in the transparency, the optical transmission of that point is sensed photoelectrically and the local intensity-time product of the printer exposure is controlled inversely. An excellent mathematical analysis of dodging printers (also called luminous masking printers) is provided in U.S. Pat. No. 3,400,632 to Wahli. As is noted by Craig, in U.S. Pat. No. 2,921,512 assigned to the assignee of this application, reproductions where the gross contrast is substantially reduced as a result of luminous masking are frequently scientifically informative but pictorially unpleasing. McIntosh, in U.S. Pat. No. 4,265,532 also assigned to the assignee of this application, indicates that electronic dodging is a tone distorting reproduction process and describes a method enabling less than maximum dodging to be obtained.
In general, for the most pleasing appearance of the reproduction, the degree of dodging selected should be that lowest amount which will enable all necessary information to be displayed with the greatest allowable tonal separation, or contrast. Heretofore, such a criterion has required knowledge of the exposure range of the receiving emulsion, an individual examination of each transparency, classification of the type of photography, and recognition of the characteristics of the reproduction process, either by contact or by projection printing.
The examination of an individual black and white transparency should produce at least five elements of information: EQU D.sub.max 1 EQU D.sub.min 2 EQU .DELTA.D=D.sub.max -D.sub.min 3 EQU D.sub.avg =(D.sub.max +D.sub.min)/2 4 EQU D.sub.sum =(1/n) (D.sub.1 +D.sub.2 +D.sub.3 . . . D.sub.n) 5
The .DELTA.D information must be compared to the emulsion exposure range so that when .DELTA.D is less, no dodging is necessary; when .DELTA.D is slightly greater, some dodging is required; and when .DELTA.D is considerably greater, maximum dodging may be needed. The D.sub.avg information represents the arithmetic average of the density extremes and it, or some function kD.sub.avg of it, is useful when reproducing pictorial subjects. Aerial photography, however, and various types of medical and scientific photography, is reproduced best when D.sub.sum is employed, because all elements of the imagery may well be of equal visual importance. Thus, the greater the number of points measured and averaged, the greater will be the probability that most photographic areas will be reproduced at or near their optimum exposure.
The Callier effect exerts an important influence over the contrast of the printing process used. Contact printers require density measurements to be made by measuring the diffused light after it has passed through the negative. Projection printers, however, require measurement of specular or projected density in order to achieve correct contrast evaluation. Applying the diffuse density measuring technique to projection printing will result in invalid measurements. Thus, it is necessary to use a different measuring technique for a contact printer than for a projection printer.
From all of the foregoing, it is clear that extraction of such a mass of image information by manual methods, although conceptually possible, is quite impractical, and that rapid preexposure evaluation of the applicable parameters of photographic transparencies requires recourse to an automatic device.